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Investigation of the Effect of Heat Upon 

the Crushing Strength and Elastic 

Properties of Concrete. 



BY 



IRA H. WOOLSON 



Authorized Reprint from the Copyriehted 

PROCBEDINGS OF THE AMERICAN SOCIETY FOR TESTING MATERIALS, 

Philadelphia, Pa. Volume V, 1905. 



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Authorized reprint from the copyrighted Proceedings of the American 
Society for Testing Materials, Philadelphia, Pa., Volume V, 1905. 



INVESTIGATION OF THE EFFECT OF HEAT UPON 

THE CRUSHING STRENGTH AND ELASTIC 

PROPERTIES OF CONCRETE. 

Compliments of 

By Ira H. Woolson. ., . ^, 

the AutJior. 

It is well known that concrete in a building construction will 
withstand the attack of a fierce conflagration for some hours and 
retain its stabihty of form and strength. This has been proven by 
actual fires in buildings, and repeated severe fire tests upon fiill- 
sized floor units and partitions. It is also well known that con- 
crete constructions have occasionally failed during conflagration 
and during official fire tests being made to determine the efficiency 
of some particular method of reinforcement. The causes of these 
failures were not alw^ays well defined. Usually they have been 
directly traceable to defective metal protection, unwise design of 
structural parts, or to the fact that the concrete was too green when 
subjected to the test. In some cases, however, the cause of failure 
was not entirely clear and much speculation has been rife as to 
just what degree of heat a concrete would stand before its strength 
and elasticity would be affected. This study was undertaken as 
an effort to throw some light upon this interesting subject. 

The first step was to ascertain what previous w^ork had already 
been done along the same line and the result obtained. A careful 
examination of the transactions of the engineering and scientific 
societies and the leading American technical journals for several 
years back furnishes very meager information. 

The fire tests of reinforced concrete, such as have been con- 
ducted by the British Fire Prevention Committee in London, and 
by the writer in co-operation with the Bureau of Buildings in New 
York City and elsewhere, have for their purpose the determination 
of three properties: ist, effect of a continuous fire at 1,700° or 
2,000° F., for three or four hours; 2d, effect of the application of 
a strong stream of water at short range while the material is still 
at a high temperature; 3d, amount of deflection due to a load 
during the fire, and subsequent increased loading to 600 lbs. per 



2 WooLSON ON Effect of Heat Upon Concrete. 

sq. ft. after the structure has cooled. The methods of construction 
and character of test are regulated by municipal specifications in 
this country, and by rules of the British Fire Prevention Com- 
mittee in England. 

The concretes used have included trap, limestone and cinder, 
and were usually 1-2-4 or 1-2-5 mixtures. The reports of numer- 
ous tests of this character were examined and in most instances the 
concrete stood the heat and subsequent loading well, but the results 
were general and referred to the quality or resistance of a particular 
construction rather than to specific data regarding the concrete 
itself. Large numbers of tests of compressive strength and elastic 
properties have been made upon concrete of various composition 
and after different preliminary treatments, but no records were 
found in which the concrete was heated prior to testing. 

The report of the U. S. Arsenal at Watertown, Mass., for 
1902, contains data of tests of neat cement cubes of several brands, 
which were heated before crushing. A synopsis and discussion 
of these is given by James E. Howard in the May, 1905, issue of 
"Cement," and some of his conclusions are given, since it is fair 
to assume that the action of neat cement under heat should be at 
least a slight, criterion of that of concrete. The following is a 
summary of the results reported by Mr. Howard. 



Table I. — Effect of Previous Heating on Crushing Strength of 
Neat Cement and i : i Sand Mortar. 

(Watertown Arsenal, 1902. J. E. Howard.) 



Composition. 












Ti 


Ultimate Crushing Strength in lbs 


. per sq. in After 


Cement 




S 


Heating. 




Temperature F 






200° 


300° 


400° 


500° 


600° 


700° 


800° 


900° 


I Alpha* 




9167 


8830 


7920 


9190 


9400 


9333 


8217 


8060 


6060 


I Alphat 




12480 


14447 


13767 


13910 


12787 


12130 


12130 


0085 




I Dyckerhof* . 




5017 








.... 


4313 


3483 


4280 




I Mankato* . . 




1867 


1657 


1876 


1966 


1603 


1453 


1496 


1400 


liSj 


I Mankatot . . . 




3873 


4043 


3523 


3810 


4133 


4133 


3057 


30OO 


2990 


I Mankato*. . . 


I 


538 


401 


432 




471 




381 




317 


I Mankatot . . . 


I 


2170 


2067 


1953 




2063 




2240 




1767 



* Ctibes set in air. 



t Cubes set in water. 



It was desired to ascertain the effect of elevation of temperature alone 
without introducing internal strains incident to a state of unequal tem- 
peratures in different parts of the specimen. The test pieces were 4-in. 
cubes cast slightly more than a year previous. Tests were at intervals 



Gift 

C^riiCiiio lust, 

24 la 



b07 



WooLsoN ON Effect of Heat Upon Concrete. 3 

of from 4 days to 4 months after heating. The cubes were gradually- 
raised to the recorded temperatures. The heating took one hour, the 
maximum temperature was held one hour, and the specimens were then 
allowed to cool slowly in dry sawdust or powdered asbestos. During 
the heating the specimens developed fine cracks ; these were hardly visible 
immediately after cooling nor were they one day later. Four daj'S after 
heating they were generally developed and at eleven days they were nearly 
at a maximum. The effect upon the crushing strength was not serious 
when the cracks were fine, as the parts fitted together under pressure. 

Table I shows the variation in ultimate crushing strength of the 
cubes. Each result is an average of three tests. This indicates that there 
is no decrease in strength up to a temperature of 600° Fahrenheit, but 
for higher temperatures the strength diminishes quite rapidly. 

A search for previous similar investigations was so fruitless, 
it was evident that our explorations were to be conducted in prac- 
tically an untrodden field, so* the method of procedure was the 
next consideration, the desire being to make the conditions con- 
form as nearly as possible to practice. 

Since all the factors which enter the concrete problem are 
variables, it is extremely difficult to arrive at even a partial solution 
under any one set of conditions. There is, first, variation in the 
quality of the cement; second, difference in size, sharpness and 
cleanness of the sand ; third, size and quahty of the stone, gravel, 
slag or cinders used; fourth, variations in the proportions of the 
three solid ingredients and the amount of water used, and, fifth, 
method of mixing and treatment after molding, including age before 
testing. This latter is quite important, for it is well known that 
the strength of concrete increases rapidly for a period of six to 
twelve months after casting and continues to increase slightly up 
to two or more years. 

It was decided to make the concrete a 1-2-4 mixture of cement, 
sand and |-inch broken stone, this being a common mixture used 
in constructing reinforced concrete floors. The cement was sup- 
plied by your Committee on Concrete and consisted of a mixture 
of different brands of the best grades of Portland. The sand was 
taken from a quantity being used in the erection of a new building 
on the University grounds. It was of medium size (90 per cent, 
passing a 12-mesh sieve), fair quality, and not especially clean. 
Two varieties of stone were employed, Hudson River blue lime- 
stone and Hudson River trap-rock. Two sets of specimens were 
prepared which were duplicates in every respect, except that one 
contained limestone and the other trap-rock. 



4 WooLSON ON Effect of Heat Upon Concrete. 

The mixing and casting were done by a laborer familiar with 
concrete work. A moderately wet mixture was used, tamping in 
the molds being continued until the surface of the concrete became 
flushed with water. 

The investigation had three primary objects. First, to ascer- 
tain at what temperature the concrete began to lose crushing 
strength due to heat treatment; second, the rate at which strength 
decreased as a result of increase of heat, and last, but not least, 
the effect of varying temperatures upon the elastic properties of 
the concrete ; the purpose being to determine if the elasticity began 
to diminish prior to the strength or concurrently with it. It was 
decided to make 500° F. the initial heat, and then to increase the 
temperature by intervals of 250° F. to 2,250° F., testing specimens 
at each temperature. The upper temperature limit was well above 
the average of a burning building, which is conservatively estimated 
by most experts as ranging from 1,500° to 2,000° F. 

The determinations of crushing strengths were made upon 
4-inch cubes; for the elastic properties, prisms 6x6x14 inches, 
were used, the height being sufficient to allow the measuring of 
compression on a length of 12 inches, and the cross-section being 
large enough to avoid the necessity of considering the specimen as 
a column. 

To establish the quality of the concrete, three cubes and three 
prisms of each composition were first tested without being heated; 
then three cubes and two prisms of each composition were tested 
at each temperature. 

Method of Heating. — The heating to 1,750° F. was done in 
an oven type of gas furnace. The furnace had a capacity of twelve 
cubes or two prisms and also allowed room for protecting them 
from the flames with fire bricks placed around the sides and top. 
The specimens were kept from contact with the floor by being 
supported on iron rods. Above 1,750° F. the heating was done in 
a large gas crucible furnace. 

To insure equal heating throughout the specimen, the rate of 
heating was arbitrarily fixed at 45 minutes to reach the first 500°, 
and 30 minutes for each successive 250°, the maximum heat being 
held 10 minutes before removing the specimen. This method sub- 
jected the prisms to a shorter period of heating than the cubes for 
every temperature except 500°, because the former were brought to 



WooLSON ON Effect of Heat Upon Concrete. 5 

the required temperature, held there, and then removed. The 
latter were charged into the furnace 1 2 at a time, brought to the 
proper heat, held there 10 minutes, and three cubes removed; 
then the temperature was raised 250°, and held again. This being 
continued until the last cubes were removed. Good results were 



Table II. — Compressive Strength of 4-Inch Trap-Rock Concrete 

Cubes. 



Specimen 
No. 



Age in 


Days. 




Between 


Before 


Heating 


Heating. 


and 




Testing. 


32 




32 


.... 


32 




36 


a 


36 


3 


36 


2 


36 


2 


36 


2 


36 


2 


36 


2 


36 


2 


36 


2 


36 


2 


36 


2 


36 


2 


50 


10 


so 


10 


so 


10 


SO 


10 


SO 


10 


50 


10 


44 


9 


44 


9 


44 


9 


44 


9 


44 





44 


9 



Heated to 
Degrees F. 



Unheated 



500 
500 
500 
7SO 
7SO 
7SO 
1000 

1000 
1000 
1250 

1250 
1250 
1500 

1500 
1500 
I7SO 
I7SO 



1750 
2000 



2000 
2250 



2250 
2250 



Ultimate 

Strength 

lbs. per sq. in. 



1903 
1913 
1892 
1808 
2100 
1853 
1880 
1690 
1950 
1547 

1273 
1418 



1163 
1459 
1265 



1602 
644 



904 

680 



790 
4S8 



626 
420 



Condition after Heating. 



Slight cracks. 

Brittle and full of min- 
ute cracks. 

Same. 

Same. 

Brittle and had several 
small cracks. 

Same. 

Same 

Few cracks ; appears 
sound. 

Sound ; no cracks. 

Same. 

Full of cracks. 

Same; one crack ex- 
tending entirely 
around. 

Full of cracks. 

Full of cracks ; one ex- 
tending around 3 
sides. 

Few cracks ; surface 
was pitted. 

Same. 

Slightly fused on one 
edge; few cracks. 

Very much fused on 
bottom. 

Full of cracks; slightly 
fused on one edge. 



obtained for the cubes for all temperatures, but it is doubtful if 
the prisms were uniformly heated at temperatures under 1,250°, 
as will be explained later. 

Temperatures were measured continuously by a Le Chatelier 
pyrometer. The thermo-couple was located about 4 inches above 
the floor of the furnace and closely surrounded by the specimens. 



6 WooLSON ON Effect of Heat Upon Concrete. 

After heating, the specimens were immediately removed from 
the furnace and allowed to cool in the air. The testing was done 
at intervals up to three weeks subsequent to the heating. 

Method of Testing. — The tests were made upon a Riehle test- 
ing machine of 100,000 pounds capacity. The cubes were faced 
on the upper surface with plaster of paris, the lower face being in 



Table III. — Compressive Strength of 4-Inch Limestone Concrete 

Cubes. 





Age in 


Days. 








Specimen 
No. 




Between 


Heated to 
Degrees F. 


Ultimate 
Strength 


Condition after Heating 


Before 


Heating 


lbs. per sq. in. 






Heating 


and 












Testing. 








I. . . . 


34 




Unheated 


1968 




2. . . . 


34 


.... 




1843 




3- .. • 


34 




" 


1640 




4. .. . 


38 


3 


500 


1227 


Somewhat brittle. 


5- .. . 


38 


3 


500 


1290 


Same. 


6 


38 


3 


500 


II 84 


Same. 


7. . . . 


38 


3 


7SO 


1122 


Brittle, and gave me- 
tallic sound if struck. 


8 


38 


3 


750 


1440 


Same. 


9. . . . 


38 


3 


750 


1170 


Same. 


10. . . . 


38 


3 


TOOO 


923 


Stone slightly calcined. 


II. . . . 


38 


3 


1000 


991 


Same. 


12. . . . 


38 


3 


1000 


1214 


Same. 


13- •• • 


38 


3 


1250 


988 


Clacination through- 
out. 


14- .. . 


38 


3 


1250 


1038 


Same, but appeared 
sound. 

Same, surface discol- 
ored. 

Same, edges chipped. 


IS 


38 


3 


1250 


903 


16 


44 


3 


1500 


680 


17. .. . 


44 


3 


1500 


778 


Same, full of small 
cracks. 


18.... 


44 


3 


1500 


838 


Same, crumbles easily. 


19. . . . 


44 


3 


1750 


832 


Same, and discolored. 


20. . . . 


44 


3 


1750 


684 


Very fragile. 


21. . . . 


44 


3 


1750 


922 




22 ... . 


44 


3 


2000 




Crumbled on cooling. 


23'. . . 


44 


3 


2000 




Same. 


24. . . . 


44 


3 


2000 




Same. 


25- . . . 


44 


3 


2250 




Same. 


26 


44 


3 


2250 




Same. 


27. .. . 


44 


3 


2250 




Same. 



all cases smooth enough to require only a few sheets of blotting 
paper to insure a firm bearing. The pressure was applied to the 
top and bottom faces as determined by their position in the mold. 
The pressure was applied very slowly and steadily until the speci- 
men failed. 

The prisms were faced on both ends with plaster and com- 
pressions measured on a gaged length of 12 inches by an electric 



WooLsoN ON Effect of Heat Upon Concrete. 7 

contact extensometer adjusted to the specimen as shown in Fig. 
I.* The load was appHed at the same rate as for the cubes. 
It is important that the load should not be appHed faster than the 
concrete will adjust itself to the new stresses, or a fictitious strength 




Fig. I. 



will be recorded and instrument readings will alter until an equihb- 
rium has been established. Compressions were measured at loads 
of o, 25, 50 and 100 pounds per square inch, and then by incre- 
ments of 200 pounds per square inch, until indications of failure 



* Acknowledgment is made to the Engineering News Publishing 
Company for the cuts used in this paper. 



r WooLSON ON Effect of Heat Upon Concrete. 

forced the removal of the instrument. Sets were measured one 
minute after the removal of each load, this interval being found 
sufficient to allow the specimen to assume a stable condition. 

It was originally intended to have the concrete at least sixty 
days old before testing, but owing to an unavoidable delay in secur- 
ing part of the material it became necessary to test the specimens 



Table IV. 



-Compressive Strength and Modulus of Elasticity of 
Trap-rock Concrete Prisms. 





Age in 
Days. 


Heated 
to 


1/5 '^ 

II 




E at 200 
lbs. per 
sq. in. 


E at 600 
lbs per 
sq. in. 


Eat 1,000 
lbs. per 
sq. in. 


Condition 

after 
Heating. 




Speci- 
men 

No. 


1 


bo 
C 

1" 


d 
c 

1 


2 . . 

3- •• • 
4. . . . 

loA . . 

iiA .. 

12B .. 

13B.. 

i6C .. 

17C .. 
20D . . 

21D .. 

24E.. 

25E .. 


33 

33 
34 
48 
48 
49 

49 

50 

50 
54 

54 

56 
56 


" "6 
6 
3 

3 

5 

5 
3 

3 

9 
9 


5oo°F. 

500° 

750° 

750° 

1,000° 

1,000° 
1,250° 

1.250° 

1,500° 
1,500° 


1.560 

1,820 
1.725 
1,404 
I 970 
1,250 

835 

735 

1,100 
910 

1,055 
250 


3,450.000 

3,180,000 

3,340,000 

715,000 

834,000 

490,000 

400,000 

128,000 

160,000 
89,000 

83,000 

19,400 


2,140,000 

3,000 000 

2,070,000 

902,000 

950,000 

526,000 

461,000 

171,500 

212,000 
122,000 

125,000 


1,700,000 

2,440,000 
1,610,000 

863,000 
1,040,000 

472,000 




Specimen in 
good condi- 
tion 


2 


Same 

Same 

Same 

Very minute 
cracks ap- 
parent .... 

Appeared 
sound, but 
brittle 

Same, had a 
metallic 
ring when 
struck 

Same 

Surface cover- 
ed with 
small cracks 

Same 

Very bad spe- 
cimen, sides 
warped and 
shattered . . 

Worse than 
24E 


riA 
12B 
13B 

i6C 
17C 

20D 
21D 

24E 
25E 



when a little over a month old. This was much to the disadvantage 
of the concrete in the fire tests, and it also accounts for the rather 
low values obtained in the tests of unheated specimens. 

Results. — Table II. gives the ultimate crushing strength of the 
4-inch trap cubes which were heated to various temperatures and 
crushed after cooling. No appreciable effect upon the strength 
can be noted until a temperature of 750° is reached. This gave 



.'♦'ooLSON ON Effect of Heat Upon Concrete. 



(^ 



slightly lower average strengths. Beyond 750° the decrease was 
marked, though there were two or three exceptions to the rule 
notable at 1,500°, where two of the specimens gave remarkably 
high results. Why these 
cubes should have with- 
stood the heat so much 
better than the others is 
not known. With the 
above excepttion, the 
surface of all specimens 
heated over 750° was 
covered with minute 
cracks. At 2,250° F. 
the cubes were slightly 
fused, due to the fact 
that fire-brick protection 
was displaced in remov- 
ing previous specimens, 
and the remainder were 
more or less exposed to 
direct contact with the 
flames. 

Table III. gives sim- 
ilar data for the limestone 
cubes. The three un- 
heated cubes show an 
average strength only 
slightly inferior to that of 
the trap mixture. Heat- 
ing to 500°, however, 
gave a great loss in 
strength. There were no 
evidences in the appear- 
ance of the cubes indicat- 
ing this deterioration. No 
further weakness resulted 
at a temperature of 750°, but beyond this the loss of strength 
continued. After heating to 2,000° and 2,250° the cubes appeared 
strong and in good condition while hot, but when cold they began 




a 



Q 



U 

o ps 

t/i o 

O en 

d 



< 



lo WooLSON ON Effect of Heat Upon Concrete. 

to disintegrate, and at the end of three days their appearance was 
as shown in Fig. 2. No attempt was made to test these specimens. 

Table IV. contains the results of the test upon the elastic 
properties of trap-rock prisms. Three curves were also plotted 
for each specimen: i, total deformation; 2, set; and 3, true 
elastic. The latter was obtained by Professor Bach's method, viz., 
by subtracting from each total deformation reading the correspond- 
ing set reading. Modulus of elasticity (E) was figured for three 
points of the true elastic curve. 

Taking the age into consideration the values for the unheated 
specimens compare favorably with the results of other investigators. 
As is usual, the value of E diminishes with increase of pressure. 
With the heated specimens this is not so marked; in fact, it is often 
the reverse, particularly with the intermediate loading. There is, 
however, a very marked decrease in the value of E due to the heat- 
ing. This change becomes very apparent even with a tempera- 
ture of 500° and the value gradually decreases with the increase of 
heat. There were some erratic results, but later investigation 
makes it quite certain they were due to imperfect heating. 

After the elastic measurements on the prisms were completed 
the extensometer was removed and the specimen loaded to failure. 
The ultimate crushing strengths which were thus obtained are given. 

Table V. gives the same data for limestone prisms. The 
moduli for the unheated specimens are about the same as those 
obtained by the writer on a series of similar tests recorded in Engi- 
neering News of June i, 1905, the average value of E obtained there 
being approximately 3,600,000 for a sand-lime-stone concrete 
55 to 58 days old, and the average here found being 3,300,000 for 
prisms 20 days younger of like composition. 

The value of E falls rapidly with increase of heat applied, 
the same as for the trap-rock mixture. 

The surfaces of the prisms of both mixtures were covered with 
minute cracks after being subjected to over 750° and then cooled. 
These cracks increased in number and size as the heat became 
higher, and at. 1,500° the prisms began to warp and disintegrate 
on cooling. This deterioration increased with time, and at the 
end of nine days one prism of each mixture was so badly crumbled 
it was unfit for test. The others were very much weakened. This 
disintegrating effect is probably due to the swelling of the cement 
as a result of recalcination. 



WooLsoN ON Effect of Heat Upon Concrete. 



II 



The curves of all the heated specimens show a large deforma- 
tion in the early part of the test when the loads were comparatively 
light. This gradually lessens as the loads increase and the middle 
portion of each curve approaches a straight line and then falls off 
again when ultimate failure begins. The large deformation at 
first is doubtless due to the closing up of the numerous fire cracks 
previously mentioned. 

Table V. — Compressive Strength and Modulus of Elasticity of 
Limestone Concrete Prisms. 





Age in 
Days. 


1 

Heated 

to 

DegreesF. 


Is 

M 

s 


E at 200 
lbs per 
sq. in. 


E at 600 
lbs. per 
sq. in. 


E at 1,000 
lbs. per 
sq. in. 


Condition 

after 
Heating. 




Speci- 
men 
No. 


bo 
c 

1 

1 

pq 





c 

a 

p. 

w 


5 

6. .. . 


30 
30 
30 

44 

44 
45 

45 

51 

51 

57 
57 

57 
57 


4 

4 
7 

7 

4 

4 

3 
3 

19 
19 


500° 

500° 
750° 

750° 

1,000° 

1,000° 


1,427 
1,452 

1,246 

1,568 

1,207 
1,110 

1,214 

932 

1,145 


3,000,000 

3,340,000 

2,500,000 

700,000 

1,330,000 
500,000 

222,000 

157,000 

172,000 

92,500 
59,000 

83,300 


1,715,000 ! 1,028,000 
2,330,000 1 2,080,000 
1,715,000 I. ^00. 000 




s 

6 

7 




7 . . . . 




8A .. 

qA.. 
14B .. 

15B .. 

18C .. 

19C . . 


352,000 

1,176,000 
333,000 

294,000 

200,000 

285,000 

13,650 
10,000 

133,000 


476,000 

972,000 
344,000 

286,000 


Specimen in 
good condi- 
tion 

Same 

Not smooth on 
sides 

Good condi- 
tion 

Stone on edge 
slightly cal- 
cined 

Same 

Stone entirely 
calcined to 
depth of 2-in. 

Same 

Stone entirely 
calcined, sides 
warped and 
shattered .. . 

Same as 24E 
to lesser de- 
gree 


8A 
9A 

14B 

15B 

i8C 
19C 


22D . . 
23D.. 

24E.. 
25E.. 


1.250° 
1,250° 

1,500° 
i>5oo° 1 


840 
740 

810 


22D 
23D 

24E 
25E 



A peculiar characteristic of many set curves for both mixtures 
is the tendency they have to reverse direction and go back towards 
the axis. A conspicuous example appears in Fig. 4. No satis- 
factory explanation for this behavior has been suggested. 

It will be noted that the elasticity of the specimens decreased 
rapidly with the increase of heat. This is clearly shown in Figs. 
3 and 4, where all three curves for each test of three typical sped- 



12 



WooLSON ON Effect of Heat Upon Concrete. 



mens of each mixture are plotted to the same scale, showing the 
character of the total deformation, set and elastic curves without 
heating, and the corresponding curves for specimens which had 
been heated to i,ooo° and 1,500° F. respectively. 







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£.1200 


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I 


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Fig. 3. — Typical Curves of Trap Concrete. 



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Compression in Inches. 

Fig. 4. — Typical Curves of Limestone Concrete. 

Figs. 3 and 4. — Typical Stress-strain Curves of Normal Concrete and 

Concrete Previously Exposed to Temperatures of 1,000 and 1,500° F. 

Test pieces 6x6 ins. 14 ins. high, of 1:2:4 concrete. Compressions 
measured on 12 -in. length. Test pieces heated at ages of 33 to 57 days, 
then cooled slowly, and tested 3 to 19 days later. 

Full lines give total deformation. 

Dotted lines give sets. 

Dot-and-dash lines give true elastic deformation = total deforma- 
tion minus set. 

The ''true elastic" curves of all the prisms tested are grouped 
in Figs. 5 and 6, which indicate very plainly the gradual decrease 
of elasticity due to heating of the concrete. 



WooLSON ON Effect of Heat Upon Concrete. 



13 



The gradual decrease in strength of both cubes and prisms due 
to heat treatment is shown by the curves in Fig. 7. In general, the 



1,600, 

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Fig. 5. — Elastic Curves of All Prisms of Trap Concrete. 




ComDression 



Incnes. 



Fig. 6. — Elastic Curves of All Prisms of Limestone Concrete. 

Figs. 5 and 6. — Elastic Curves of Normal Concrete and Concrete Pre- 
viously Exposed to Various Temperatures from 500° to i,5oo°F. 



Test pieces 6x6 ins., 14 ins. high, of 1:2:4 concrete. Compressions 
measured on 12 -in. length. Test pieces heated at ages of 33 to 57 days, 
then cooled slowly, and tested 3 to 19 days later. 

Two specimens tested for each temperature. 

All curves give true elastic deformation =total deformation minus set. 



14 WooLSON ON Effect of Heat Upon Concrete. 



2,000 

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Tempera+ures in Degrees F. 

Fig. 7. — Curve of Crushing Strength. 



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Fig. 8. — Curve of Modtilus of Elasticity. 

Figs. 7 and 8. — Variation of Crushing Strength and Elasticity of Concrete 

With Temperature of Previous Heat Exposure. 

Test pieces 4-in. cubes, 6x6 ins. x 14-in. prisms; all of i : 2 : 4: con- 
crete. 

Curves of elasticity give the modulus at unit stress of 200 lbs." 
per sq. in. 



WooLsoN ON Effect of Heat Upon Concrete, 



15 



trap-rock mixtures were the stronger. There were some irregu- 
larities, which undoubtedly resulted from defective heating. Fig. 
8 shows graphically the drop in the value of E for both mixtures 
due to heating. 

As stated in the early part of this paper, the length of time a 
specimen should remain in the furnace to bring it to a certain heat 
was fixed beforehand, and with the 4- inch cubes the rate of heating 
employed appeared to give them uniform treatment throughout. 

It was supposed the prisms would heat uniformly also in the 
time allowed, but subsequent results raised doubts regarding this. 
However, owing to the delays previously mentioned it was now too 
late in the season to duplicate the specimens and get them tested 
this spring, so the tests were completed and this report is rendered 
upon the data obtained. 

Table VI. gives the actual times of heating for each class of 
specimen. It is known now that the time allowed for the prisms 
was not nearly sufficient to insure uniform temperature through- 
out. This is particularly true for the low temperatures. With 
the high temperatures the variation was probably not so great. 

A test was recently made of the conductivity of the concrete in 
the prisms under the conditions of heating employed in the tests. 

Table VI. — Period of Heating the Test Specimens. 



Specimen. 


Heated to 
Degrees F. 


Time. 




500° 

750° 

1,000° 

1,250° 

1,500° 

1.750° 

2,000° 

2,250° 

500° 

750° 

1,000° 

1,250° 

T,500° 


55 minutes. 

1 hr., 35 min. 

2 " 15 " 
2 " 55 " 

2 " 55 " 

3 " 35 " 

3 " 55 " 

4 " 35 " 
55 mmutes. 

I hr., 25 min. 

1 " 55 " 

2 " 25 " 
2 55 " 




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and it was found that by allowing i hour 15 minutes to bring the 
furnace temperature up to 750° F., and then holding that tempera- 
ture constant, it required 2 hours 40 minutes more for the interior 
of two different prisms to attain the same temperature. Then 
raising the furnace temperature to 1,000° in 30 minutes, it required 
I hour 10 minutes more for the prisms to become uniformly heated 
throughout. The tests were made by imbedding thermo-couples 



1 6 WooLsoN ON Effect of Heat Upon Concrete 

in the middle of the prisms, and connecting them by switch to the 
same galvanometer on which the couple in the furnace was record- 
ing. The concrete on which this test was made was 28 days old. 
In this instance it required 5 hours 35 minutes to obtain a tempera- 
ture of 1,000° F. through 3 inches of concrete where the specimen 
was surrounded by heat on all sides, with no radiation possible. 

This last experiment proved that the concrete had a very low 
conductivity and made certain the fact that the prisms tested had 
not been heated throughout to the temperature with which they are 
credited. At the same time, the writer believes, it explains the 
apparent discrepancy which exists between the results of these 
tests and some very satisfactory fire tests which have been made 
upon full-sized floor constructions. In a test of the latter character 
the fire is applied to one side only, and, although a heat of 1,700° 
to 2,000° is maintained for four hours, the concrete is such a poor 
conductor of heat that only a small portion of it ever reaches a 
temperature which would cause it to deteriorate to any great extent. 

The writer is fully aware that the data presented are very in- 
complete and by no means sufficient upon which to base conclu- 
sions. The problem is an extremely difficult one, and at the same 
time very important. He plans to continue the investigation as 
opportunity offers and hopes that other investigators will give it 
their attention, so that in the near future some really reliable data 
may be obtained. 

The investigation of this subject, together with the experi- 
ments detailed in this paper, was undertaken at the request of the 
Joint Committees on Concrete and Reinforced Concrete of this 
Society, and forms part of the broad inquiry into the properties of 
that material which is being conducted in the various testing 
laboratories and technical schools of the country, under the general 
supervision of that Committee. 

The actual work of the investigation, together with the calcu- 
lations and plotting of the curves, was done under direction of the 
writer by Messrs. F. H. Burch, Jr., and W. H. Council, Jr., two 
students in the Department of Mechanical Engineering at Colum- 
bia University, and formed the subject of a research thesis problem 
in their senior year. Much credit is due these men for their faith- 
ful devotion to the work, and their energy in performing the tedious 
details. The presentation of this paper at this time was made 
possible only by their conscientious assistance. 



DISCUSSION. 



Mr. R. W. Lesley. — This paper seems to involve two impor- Mr. Lesley, 
tant questions: one, the chemical character of cement, and the 
other going to the strength of beams and floor construction exposed 
to fire. The general proposition is one which should bring a good 
many engineers to their feet. 

Mr. William Kent — There is one practical question I Mr. Kent. 
should like to ask. Suppose a warehouse is built of concrete, 
with a factor of safety of four, and there should be a fire in the 
combustible material stored in the warehouse — suppose, now 
there is reason to suspect that while the building did not fail, and 
the fire was put out, still it lasted such a long time that the concrete 
might have been heated to 1,500° F., and that the factor of safety 
of the building is possibly reduced to below two. Should the build- 
ing be condemned and rebuilt, so far as the floor and the parts 
sustaining the pressure of the load are concerned? 

Mr. Lesley. — The Chair will answer that from a practical Mr. Lesley, 
experience. In Belgium there was a fire in a large knitting mill 
constructed under the Hennebique System entirely of concrete. 
Under the contract it was provided that certain tests of the floors 
and building should be had before acceptance. All the require- 
ments were met and the owners took possession. Subsequently, 
there was a fire in the mill, the machinery was badly injured, some 
of the shafting twisted and a great deal of combustible matter was 
destroyed. The mill itself, however, was not injured in any way. 
Before starting up again, the owners, who held a guarantee from 
the Hennebique Company, wanted to assure themselves that the 
mill walls, floors, etc., still met the requirements of the original 
specifications, and that the Building Laws of Belgium had been 
comphed with. Of course, the mill was a little bit older than when 
it was first constructed, and at the inspection after the fire it was 
ascertained that there was a gain of 20 per cent, in strength in every 
one of the required tests. Possibly that answers the question. 

Mr. Richard L. Humphrey. — I think the tests that Prof. Mr. Humphrey. 



1 8 Discussion on Effect of Heat Upon Concrete. 



Mio Humphrey. Woolson has just described to us are extremely interesting and 
valuable and open up important fields in the investigation of prop- 
erties of cement. It has given rise to a number of thoughts which 
I should like to offer by the way of comment. I understood the 
tests were made on cubes about sixty days old. 

Mr. Woolson. Mr. Ira H. Woolson. — Thirty to forty days. 

Mr. Humphrey. Mr. HUMPHREY. — In Studying the effect of heat on hardened 

mortar or concrete, it is necessary to understand the process of 
hardening. From the moment the clinker is reduced to an im- 
palpable powder, until it is finally hydrated, it is constantly under- 
going changes produced by the reactions which tend to convert it 
from an unstable to a stable compound. It is doubtful whether 
these changes ever cease. The process of hardening in a cement 
is one of hydration resulting from the water which is added to it or 
which is absorbed from the air. The process of hardening in a 
mortar or a concrete is due to the crystalhzation or hydration of 
the cement which, during the mixing has been forced into the 
pores in the surfaces of the sand, gravel, stone, etc., in a plastic 
condition. This is the bond which holds the sand, stone or gravel 
together, and is destroyed at a temperature sufhciently high to drive 
off the water of crystallization. 

The effect of heat, therefore, is one of degree, depending on 
the age of the concrete or mortar and on the intensity and duration 
of the heat. If the heat conditions are right the mass will be 
eventually reduced to its original state, prior to hardening. The 
larger the mass the more slowly is this water driven off, as it requires 
time for the water from the interior to reach the surface. 

In addition to this water of hydration, there is also present 
in the void spaces of the mortar or concrete a certain amount of 
entrained or hygroscopic water, which, under the action of heat, 
expands, with a force sufficient to disrupt the mass — especially if 
very green. The amount of water will depend on the porosity, 
a-nd decreases, therefore, as the density increases. 

Green concrete when subjected to the action of heat, under- 
goes a sweating process in which this entrained water is gradually 
brought to the surface, this phenomenon disappearing with the 
absorption of the water either by the air or by the concrete in 
hardening. The expansion of this water under heat, which draws 
it to the surface, frequently disrupts the mass. 



Discussion on Effect of Heat Upon Concrete-. 19 

Professor Woolson's tests were made on six-inch cubes of Mr. Humphrey, 
green concrete. The superficial area of a six-inch cube bears a 
large ratio to the mass. The action of fire on such a test piece 
would be much greater than would be the case with a larger mass, 
such as the floors and walls of a building where the action would 
be largely on one surface only, and while this surface might be 
damaged by fire, it would not extend sufficiently into the mass as 
to seriously affect the strength. It would seem, therefore, that, 
while Professor Woolson's experiments show us what occurs in 
the case of small cubes of green concrete submitted to an intense 
heat, such information to be of practical value should be carried 
on with large masses of concrete which have hardened for six or 
more months. I believe in that event we would find nothing like 
the destructive action found by Professor Woolson. These fire 
tests are usually much more severe than likely to occur, because 
it is doubtful whether such conditions would ever be obtained in 
an actual conflagration. If the fire were sufficient to develop a 
red heat in the concrete, it is more than likely that the sur- 
rounding temperature would be such that it would be impossible 
to get near enough to turn on a stream of water, as the water 
would be volatilized before it could reach the surface of the 
concrete. 

Whether a building should be torn down after a severe fire, 
as suggested by Mr. Kent, is dependent on the damage which has 
been done, which can be determined by suitable inspection just 
as in the case of any other building There are many examples 
of buildings of concrete which have successfully passed through 
severe fires. 

A notable instance is the plant of the Pacific Borax Company 
at Bayonne, New Jersey. This plant consisted of a four-story 
building of reinforced concrete with wooden roof, door and window 
frames, as well as wooden posts for supporting two tanks of large 
capacity on the roof. Adjacent was a single- stor}^ building with 
reinforced concrete walls, covered partly by a wooden and partly 
by a corrugated iron roof, the latter being supported by a skeleton 
of steel. On each floor there was inflammable material which was 
practically consumed, as was all the wood work, including the 
twelve-inch wooden posts supporting the tanks on the roof. The 
steel skeleton completely collapsed, the columns folding up hke a 



20 Discussion on Effect of Heat Upon Concrete. 



Mr. Humphrey, ribbon. After the fire the building was cleaned, and the concrete 
appeared to be in good condition, showing no evidences of 
damage. 

The character and age of the concrete has much to do with its 
fire-resisting quahties; the more dense and the older the mass the 
less water will be absorbed and entrained. Therefore a thoroughly 
hardened dense mass of mo. tar or concrete is likely to suffer little 
if any damage from the effects of fire. 

It is to be hoped that Professor Woolson will continue his 
investigations on larger masses of older concrete. 
IMr. Sabin- Mr. L. C. Sabin. — One of the most important points brought 

out in this very instructive paper is the fact that it required nearly 
six hours for a temperature of i,ooo° F. to penetrate three inches 
of concrete. It is probable that the concrete used in these tests 
was of a better grade than is usually employed, and thus while 
stronger, it was also less porous, and, therefore, its conduc- 
tivity was greater than the concrete ordinarily used in building 
construction. 

The fact that the modulus of elasticity decreases more rapidly 
as the result of the application of heat than does the compressive 
strength is also an interesting one, for it means that a larger share 
of the load on a beam is transferred to the reinforcement reheving 
the concrete when the latter becomes weakened. 

During this meeting I have been told, by someone whose name 
I do not now recall, of some experiments in which briquettes were 
heated and some broken while others were replaced in water and 
regained a portion of their lost strength. I am sure it would be 
interesting if my informant would say something about these 
experiments. 
Mr.'Lazeii. Mr. E. W. Lazell. — In our laboratory briquettes of standard 

size made from three different brands of cement were exposed 
to a temperature of between i,ooo° and 1,200° F., in such a man- 
ner that the flame did not come in actual contact with the bri- 
quettes. The briquettes were held at this temperature for a period 
of six to eight hours and then allowed to cool down slowly. 

A part of each lot of briquettes were broken w^hen they were 
cold, and these gave practically no strength. The remaining 
briquettes of each lot were then immersed in water and were 
broken after having been in water 7 and 28 days respectively. 



Discussion on Effect of Heat Upon Concrete. 21 

The tensile strength resuhs obtained from the 28-day tests Mr. LazeU. 
were practically the same as those obtained from similar bri- 
quettes which had not been subjected to heat. This pointed to 
the conclusion that cement which has been de-hydrated at the 
temperature of the experiment with a loss of strength, regains 
its strength on being immersed in water 28 days, thus indicating 
re-hydration of the cement. 

Mr. Sanford E. Thompson. — The size of the specimen Mr. Thompson, 
largely affects the result. We all know that the surface of the 
concrete is disintegrated by fire to depths, var}'^ing with the con- 
ditions, from J to I J in. If the surfaces of a 4-in. cube are dis n- 
tegrated to the depth of J in. and this surface concrete rendered 
practically of no strength, the area of compression is reduced from 
16 sq. in. to 9 sq. in., that is, nearly one-half, and the strength may 
be expected to decrease in the same ratio. If we assume the con- 
crete to be affected to the depth of i in., its area of horizontal sec- 
tion is reduced from 16 sq. in. to 4 sq. in., or to one-fourth the origi- 
nal area. At a temperature of 1,500° F., a disintegration of at least 
J in. would be expected, and this of itself would explain the 50 
per cent, reduction in strength shown in the diagram. 

One of the most important subjects in fire resistance of con- 
crete, and one which has never been satisfactorily investigated, is 
the comparative value of dift'erent rocks for the coarse aggregate. 
The question has often been raised whether a limestone mixture 
would not be changed to lime and the strength of the concrete 
utterly destroyed. While in these experiments the limestone mix- 
ture did not carry as high a load as the trap-rock concrete, it cer- 
tainly did stand, when the reduction of area is considered, a very 
high stress throughout the test. 

Mr. G. p. Hemstreet. — I should Uke to ask a question about ^j.. Hemstreet. 
the hmestone and also make a little suggestion. Hudson River 
Hmestone is found on both sides of the river. That from the west 
side, which is now marketed in the greatest quantity in New York 
City, contains comparatively little carbonate of lime, and it is a 
very hard blue stone. That on the east bank contains a very large 
percentage of carbonate of lime and is a soft white stone. I should 
like to ask if it is known which limestone was used. 

Mr. Woolson. — Tompkins stone. Mr. Wooison. 

Mr. Hemstreet. — That contains very Httle carbonate of Mr. Hemstreet. 



22 Discussion on Effect of Heat Upon Concrete. 

Mr. Hemstreet lime, and is a very hard blue stone. Another thing I should hke 
to suggest would be the effect of moderate heat apphed for a long 
length of lime. I happen to know of several concrete chimneys 
lined with fire brick for a portion of their height. If they should 
be subjected to a temperature of 600 to 800° F. for several hours 
a day and for several years, is that concrete going to gradually 
become weakened by the effect of the moderate heat for a long 
length of time ? I think if we could have some concrete cubes put 
in a chimney and kept there for a year, it might be an interesting 
experiment. 

Mr. Cummings. Mr. Robert A. CuMMiNGS. — In connection with the fire 

resistance of concrete I should like to record a little experiment 
I made last year in Pittsburg, A gentleman interested in the utih- 
zation of the waste products of blast furnaces brought me two 
sample bricks, one composed of Portland cement and sand from 
slag granulate, and the other of Portland cement and ordinary 
river sand, both mixed in the proportion of one to three. He 
wanted my opinion of their relative values for a building. 

There was but slight difference in appearance and strength. 
But I placed them in the hot coal furnace of the boilers in the base- 
ment of our office building. In fifteen minutes the brick composed 
of river sand and cement had become disintegrated and partly 
fused. I was unable to take it out of the furnace except in the shape 
of a clinker. At the same time the slag sand brick had not yet 
gotten beyond an ordinary red heat and it took about half-an-hour 
to reach a white heat. While at a white heat I took it out of the 
furnace and placed it in a pail of ice water. The effect was sur- 
prising as only the edges of the brick crumbled. 

I think it is an interesting experiment and indicates that the 
waste product of the blast furnace in the shape of slag is a good 
material for firep roofing. I do not know the exact age of the bricks, 
but it was probably six months. 

Mr. Lesley Mr. Lesley. — There are well-known results that have 

occurred in large fires which are practical tests of fire-proofing 
material. There have also been scientific tests conducted under 
the most skilful methods in a large way to determine this same 
fact. We all remember that the whole world was thrown into a 
turmoil some years ago by some laboratory experiments as to the 
effect of salt water upon Portland cement. It was feared that all 



Discussion on Effect of Heat Upon Concrete. 23 

the docks and piers and ever^^thing built of Portland cement would Mr. Lesley, 
fall a\Yay and would be disintegrated. In point of fact, the experi- 
ments disintegrated, but the docks and piers stood. 

The best illustration of this latter state of facts is shown in a 
series of papers read before the Engineering Congress, at St. Louis, 
by two Japanese engineers, who built some enormous harbor 
work, which took five or six years to finish. The docks are still 
standing.. The writer drew an absolute distinction between the 
laboratory results of salt water experiments, and the actual results 
of work done in thousands and thousands of cubic yards of this 
material. 

Now you have another illustration which goes to the effect of 
two different natural forces on concrete. One is the effect of frost, 
and the other is the effect of heat on small bodies of concrete. We 
all know that if you take a small, or even a large, body of concrete, 
the first thing that happens is that what we call a chemical action 
is set up, and these little needles — angular pieces — get together 
and do what we call setting. The moment that is under way along 
comes a powerful influence known as Jack Frost, who stops that 
operation and sets up another one, which takes the water which 
is necessary for this chemical action to go on, and turns it into 
little crystals, hardening the entire mass, thus suspending chemical 
development, which again begins when the mass thaws. Now, 
it may be that the surface of that concrete has been injured, but 
that does not in any way destroy it as a whole. 

So, also, if you put concrete to the test of fire and drive out the 
water the surface may be injured, but the surface that is injured . 
is so small in proportion to the amount of concrete actually used 
that it can always be discovered, and in fact no harm results. In 
a small cube there is a great deal of surface destruction propor- 
tionate to the mass, but in a large mass the destruction of the sur- 
face, either by frost or heat, is so light that it will not destroy the 
reliability of the mass. 

Those are two points that seem to come to my mind on this 
question. 

Therefore, if we take a mass of cement which is in the act of 
setting, for we are all aware that cement or concrete does not get 
its final set before very long periods, and that the water in the mass 
is producing the chemical action, it would be readily agreed that 



24 Discussion on Effect of Heat Upon Concrete. 

Mr. Lesley, all the chemical action has not yet been completed at the expira- 
tion of so short a period as 30 days. Consequently, if small cubes 
of cement, in which the chemical action is not yet finished, are 
exposed in a fiery furnace at the end of so short a period as 30 days, 
it is a grave question whether, first of all, enough water is not driven 
off in the first few moments to deprive the mass, which is endeavor- 
ing to go on and complete its chemical action of setting, of the 
means so to do ; or, in other words, if by driving off the water at so 
short a period and in such small bodies, the mass of concrete 
has not been deprived of its means of livelihood. 

Those familiar with the testing of long time se1;s of briquettes 
can thoroughly appreciate how long it is before the actual final 
setting of the cement is completed. Sometimes a little white dry 
spot is found in the center of the briquette at the end of a year or 
eighteen months, and it may be three or four years before this little 
white mass extends over the whole surface of the briquette, show- 
ing that no more water is admitted into the interior, and that setting 
has finally been completed. 

Mr. Patton. Mr. Alfred G. Patton. — As a member of the National Fire 

Protection Association I am very much interested in this discussion. 
We know that the resistance of steel is reduced very quickly 
as its temperature rises. In our laboratory at Chicago numerous 
tests were made as to the penetration of heat into the cement 
mass, and the effect of the heat on the reinforcement. We know 
that the floor load is carried by the reinforcement. The tensile 
strength or cohesive properties of the cement not being sufficient 
to carry any considerable floor load. The moment the tempera- 
ture of the steel is raised to any extent, it reduces the resistance of 
the steel below the factor of safety and something is going to happen. 
We find that steel raised to a temperature of 1,400° F. only retains 
about 10 per cent, of its resistance. The question is, how are we 
going to protect that steel from the action of the flames; not how 
far is the concrete going to resist their action. There have been 
buildings constructed of cement that failed under the fire test. 
In Baltimore there was a bank building that failed very decidedly. 
There were other structures, however, that stood the test of the 
fire. In one case the whole building was gutted, and the floor 
arches, beams and columns stood with practically no deteriora- 
tion. Now there was some reason for that, and in our Committee 



Discussion on Effect of Heat Upon Concrete. 25 

work some have come to the conclusion that, perhaps, the reason Mr. Patton. 
was from some inherent quahty of the cement in one case, which 
permitted the heat to attack the steel work, and in the other case 
protected the reinforcement to such an extent that the arches failed 
to yield. It seems to us that there is greater necessity for protect- 
ing the reinforcement than worrying about the concrete. 

I am glad to learn from Professor Woolson's experiment that 
cement is such a good non-conductor. We made a series of tests 
in Chicago, taking some beams, I think about 8'^ x 11 f' section 
through which three steel rods were put, one i in. from the bottom, 
another 2 in. from the bottom, the third, 3 in. from the bottom. 
Through holes bored into the tops of the beams thermometers 
were inserted, which rested on the steel rods to note the rise in 
temperature of the rods. A temperature of about 2,000° F. was 
maintained under the beams for four hours. We found that the 
temperature of the steel where there was only i in. of concrete 
covering rose to the danger point, in a very short period. Where 
there were 2 in. of covering much better results were apparent. 
I am requested by Mr. Hexamer, President of the National Fire 
Protection Association, and by the Chairman of our Committee, 
to express to you our sincere sympathy in your investigations, and 
our earnest desire to co-operate in any way to ascertain the true 
possibility of cement as a factor in fire-proof construction. 

Mr. Leonard C. Wason. — I just want to bring out one Mr. Wason. 
thought, epecially in answer to the query of Professor Kent. In 
the designing of most columns which I have had to make, after 
determining the cross-section necessary for the strength, I have 
added about i in. on a side. By observation of the various fire 
tests which I have had an opportunity to see, I found the damage 
caused by the heat did not extend more than an inch into the sur- 
face. Therefore even if a column should lose i in. of surface by a 
severe fire, it still has in its center sufficient strength to carry the 
load, and that surface can be repaired by plaster without detriment 
to the entire construction. The attack of the heat is usually less 
severe on the columns than on the horizontal surfaces on the under 
side of floors. Therefore, the columns are not as likely to be 
damaged as the floor they support. 

^Ir. Rudolph P. Miller. — The New York Building Code Mr. Muier. 
prescribes a test for floors, which is designed to bring out the fire- 



26 Discussion on Effect of Heat Upon Concrete. 

Mr. Miller, proof qualities of the materials entering into the construction. In 
framing the law, the protection of the steel frame structure now 
in general use was the main object aimed at. Reinforced con- 
crete was practically unknown at the time the present law went 
into force. However, in testing reinforced concrete construc- 
tions, the requirements are followed as closely as possible. The 
prescribed test is essentially as follows: 

A test structure of masonry, about fourteen feet wide by 
twelve to twenty feet long, of which the construction to be tested 
forms the roof, is erected. The necessary chimneys, draught 
openings, firing door and grate are provided. In this structure 
a fire of an average temperature of 1,700° Fahrenheit, determined 
by pyrometer, is maintained for four hours. At the end of that 
time, water at a pressure of sixty pounds is applied to the under- 
side of the construction for five minutes, through a one and 
one-eighth-inch nozzle; then the top is flooded, and the sixty- 
pound stream is again applied to the underside for five min- 
utes. During this test the construction carries a load of one 
hundred and fifty pounds per square foot, for which it is de- 
signed. After the application of fire and water, the load is 
increased to six hundred pounds per square foot. The con- 
ditions of approval are that no fire or water shall have passed 
through the construction, the load shall have been safely sustained, 
and the permanent deflection after the removal of the final load 
shall not exceed two and one-half inches. This test on full-size 
constructions no doubt has many advantages over the laboratory 
test described in the paper, though compared with them it is 
perhaps a crude test. 

Since September, 1896, more than fifty such tests have been 
made under the supervision of the New York Building authorities, 
the majority of which were on cinder concrete construction. The 
practically uniform results in these cases have estabhshed beyond 
doubt the thoroughly fireproof character of the cinder concrete 
construction. 

The results on the stone concrete have not been so satisfac- 
tory. Out of fourteen tests eight were successful. The failures 
were due probably in most cases to the fact that the test was made 
too soon, before the concrete had set sufliciently. In all the tests 
on stone concrete, water was driven off in great quantities during 



Discussion on Effect of Heat Upon Concrete. 27 

the first half-hour. When the concrete has not had a chance to Mr. Miller, 
dry out sufficiently the expulsion of the water and conversion into 
steam are likely to disrupt it. Even in the successful tests the con- 
crete was flaked off on the surface exposed to the fire, for a depth 
of about one inch. It, therefore, becomes of great importance 
that a sufficient thickness of concrete shall be provided around 
the metal reinforcement. 

Another matter that deserves some attention is the time that 
should be allowed for the setting of the concrete before the test 
is made. A maximum limit should be fixed dependent on the 
interval between construction of a building and its occupancy 
in ordinar)^ cases. New York practice in this respect was for- 
merly thirty days, but more recently a longer time has been given 
before testing. 

AIr. Woolson. — There are just one or two points brought out Mr. Wooison. 
by Mr. Miller that I wish to speak about. With regard to water 
in the concrete : where the test was made in thirty days, we have 
repeatedly noted in large floor tests, that the water has come out 
to such an extent that on a floor of 15-ft. span we have had any- 
where from J to I in. of water on top during the first two hours of 
the test. That water came up out of the concrete. Whether it 
was held there mechanically or was water of crystallization dis- 
associated by the heat, I do not know, but it is quite evident that 
the concrete was too green. This accumulation of water was 
particularly marked where the concrete stood during the early 
spring months and was thoroughly wet. The older the concrete, 
the better results we get. The only question is, what the age 
limit should be at which we should test it. 

Now, a word with regard to Mr. Kent's quer}^ I might 
say that we have made repeated tests upon iull-size floor units 
that Mr. Miller has spoken of, where we applied the heat 
for four hours, var}dng from 1,700 to 2,000° F. After the build- 
ings cooled we loaded them again to 600 lbs. per sq. ft., and 
they carried the load successfully. That would seem to show 
that there was more than 50 per cent, of strength left in the 
concrete. In my judgment, the resistance to heat comes from the 
non-conductivity of concrete .The upper part of the floors were 
injured to a very sHght extent. We found that water on red-hot 
concrete would break off some of the surface. In some cases, 



28 Discussion on Effect of Heat Upon Concrete. 

Mr. Wooison. where girders were used, it knocked the concrete from the bottom 
part of the girder, and exposed the metal. A subsequent examina- 
tion of buildings after the ten minutes application of water which 
we employ in our tests, showed the concrete was much better 
where the water had been applied than where it had not. 

In the tests for conductivity it took about four hours to pene- 
trate to the middle of a 6-in. block ; in other words, to go through 
3 in. of concrete where there was no possible chance for radiation, 
and the heat was coming in on all sides. 

With regard to the sand and cement mixture that has been 
spoken of. In our testing station we have a building 14 ft. long, 
by 9 ft. wide and 9 ft. high, which is used for testing partitions. 
The ceiling and end walls are permanent; the side walls are 
removable, and new partitions are put in. The ceihng and end 
walls are made of a 1:4 mixture of sand and cement, 4 in. thick. 
We have had eight or nine tests in that building, averaging i,7oo°F. 
for half of the time, and gradually approaching that during the 
rest of the time. Those walls are still in excellent condition; 
good for an indefinite number of tests, as far as I can see. I shall 
have to move the building soon and must tear it down; I am 
regretting the job I have on my hands, it is in such good condition. 

The question has been raised regarding laboratory tests 
not being comparable with large practical tests. I agree thoroughly 
on that point. The only idea of these tests was to ascertain, if 
possible, what the effect on the specimen of concrete would be 
as to its elasticity and its strength, provided we could give it a 
uniform heat all through. That is all these tests bring out. They 
do not affect, I think, the general problem of concrete as a mass 
construction. 



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